Japanese patent Kokai publication No. Hei 5-290793 discloses a photomultiplier tube includes an electron multiplier accommodated in a hermetically sealed vessel. The vessel has a metal side tube having a flange-shaped upper end. The flange-shaped end is welded to an upper surface of a faceplate, thereby ensuring airtightness of the vessel.
Referring to FIG. 10, the conventional photomultiplier tube described above has a flange 101 bent inwardly at the whole upper end of the side tube 100. The flange 101 results in the decrease in an effective sensitive area of a faceplate 102. If the faceplate 102 has a size of 50 mm×50 mm, and the flange 101 having a 1.5 mm width is fixed at and around a periphery of the faceplate 102, the sensitive effective area is clearly considered 88%. This type of photomultiplier tube has succeeded in obtaining more than 80% of a sensitive effective area. Recently, many photomultiplier tubes have been used and arranged as a unit for their applications. In those applications, photomultiplier tubes are required to have substantially 100% of sensitive effective area. In other words, photomultiplier tubes having substantially no dead sensitive area is necessary. As long as the side tube 100 and the faceplate 102 are joined by crimping the flange 101, a problem arises that the photomultiplier tubes have more than 10% of dead sensitive area. If a lot of conventional photomultiplier tube are juxtaposed densely, a substantial dead sensitive area may be easily produced. Japanese patent Kokai publication No. Hei 5-290793 discloses that the side tube 100 and the faceplate 102 are joined without using a flange. In the photomultiplier tube disclosed, the faceplate 102 is just in contact with an edge of the side tube 100. This publication has no disclosure on how to join the side tube 100 and the faceplate 102. As described above, a leak may occur from the hermetically sealed vessel, when the faceplate 102 is merely in contact with the edge of the side tube 100.